Radial antenna and plasma device using it

ABSTRACT

A plasma device includes a first conductive plate ( 31 ) in which a plurality of slots ( 36 ) are formed, a second conductive plate ( 32 ) having a microwave inlet ( 35 ) and disposed opposite to the first conductive plate ( 31 ), a ring member ( 34 ) for connecting peripheral edges of the first and second conductive plates ( 31, 32 ), and a conductive adjusting member ( 37 ) provided on said second conductive plate ( 32 ) within a radial waveguide ( 33 ) formed by the first and second conductive plates ( 31, 32 ) and serving to adjust a distance (d1, d2) up to the first conductive plate ( 31 ). With this arrangement, a desired electric field radiation distribution can be obtained without inducing abnormal discharge.

BACKGROUND OF THE INVENTION

The present invention relates to a radial antenna and a plasma deviceusing it.

In the manufacture of a semiconductor device, plasma devices are usedoften to perform processes such as formation of an oxide film, crystalgrowth of a semiconductor layer, etching, and ashing. Among the plasmadevices, a microwave plasma device is available which produces ahigh-density plasma by introducing a microwave into a processing vesselthrough a radial antenna. As a characteristic feature of the microwaveplasma device, it has wide applications because it can stably produce aplasma even if the pressure is comparatively low.

FIG. 7 includes views showing the arrangement of an example of a radialantenna conventionally used in the microwave plasma device, and thedistribution of its electric field radiation. FIG. 7( a) is a conceptualview showing the radiation surface of the radial antenna, FIG. 7( b) isa sectional view taken along the line VIIb-VIIb′ of FIG. 7( a), and FIG.7( c) is a conceptual view showing the distribution of the electricfield radiated by the radial antenna. In FIG. 7( c), the axis ofabscissa represents the distance from the center of the radial antennain the radial direction, and the axis of ordinate represents thestrength of the electric field radiated from the radial antenna. FIG. 8is a view showing the shape of a slot formed in the radiation surface ofthe radial antenna shown in FIG. 7.

As shown in FIG. 7( b), a radial antenna 230 conventionally used in theplasma device is formed of two parallel conductive plates 231 and 232which form a radial waveguide 233, and a ring member 234 which connectsthe peripheral edges of the conductive plates 231 and 232. A microwaveinlet 235 is formed at the center of the conductive plate 232 tointroduce a microwave from a microwave generator (not shown). Theconductive plate 231 also has a large number of slots 236 to radiate themicrowave propagating in the radial waveguide 233 to a processing vessel(not shown). When the influence on the electromagnetic field in theradial waveguide 233 is considered, the smaller a width W2 of each slot236, the better. If, however, the width W2 is excessively small, it maycause abnormal discharge. Thus, the width W2 is usually set to about 2mm (W2≦λg/4 where λg is the wavelength of the microwave in the radialwaveguide 233).

The microwave introduced from the microwave inlet 235 propagatesradially from the center toward the peripheral portion of the radialwaveguide 233. As the microwave is radiated little by little from thelarge number of slots 236, the power density in the radial waveguide 233gradually decreases toward the peripheral portion of the radialwaveguide 233. The electric field radiation efficiency of the slots 236gradually increases as their length or slot length L2 increases from 0(zero), and reaches the maximum when the slot length L2 corresponds toλg/2.

Under these conditions, in order to obtain the radiated electric fielddistribution as shown in, e.g., FIG. 7( c), conventionally, the slotlength L2 was adjusted, so the radiated electric field strength wascontrolled. More specifically, the further away from the center of theconductive plate 231, the larger the slot length L2, as shown in FIG. 7(a), so the slot length L2 at the peripheral portion where the powerdensity was small was set close to a length corresponding to λg/2, thusrealizing the radiated electric field distribution as shown in FIG. 7(c).

When, hover, W2=λg/2 where the electric field radiation efficiency ofthe slot 236 becomes maximum, the microwave resonates. Particularly,when the width W2 of the slot 236 is as small as 2 mm, abnormaldischarge is induced. When this discharge heats the portion around theslot 236, the surrounding portion of the slot 236 is distorted, orstarts to melt.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problems, and hasas its object to obtain a desired electric field radiation distributionwithout inducing abnormal discharge.

In order to achieve the above object, a radial antenna according to thepresent invention is characterized by comprising a first conductiveplate in which a plurality of slots are formed, a second conductiveplate having a microwave inlet and disposed opposite to the firstconductive plate, a ring member for connecting peripheral edges of thefirst and second conductive plates, and a conductive adjusting memberprovided on the second conductive plate in a radial waveguide formed bythe first and second conductive plates and serving to adjust a distanceup to the first conductive plate. When the distance from the secondconductive plate to the first conductive plate is decreased, the powerdensity between the first and second conductive plates can be increased.Thus, even when the slot length of slots formed in the first conductiveplate is decreased to be sufficiently smaller than a lengthcorresponding to λg/2, the radiated electric field strength can beincreased.

A plurality of conductive adjusting members may be disposed radiallywhen viewed from the top. Alternatively, one or a plurality of adjustingmembers may be disposed along the periphery of the second conductiveplate.

The conductive member may be set to become higher as it is further awayfrom the center of the second conductive plate. Then, the power densityin the radial waveguide in the radial direction can be changed.

In the radial antenna descried above, when the slots are substantiallyrectangular or arcuate, the slot length is preferably decreased to besmaller than a length corresponding to λg/3. Then, even if the widths ofthe slots are narrow, induction of abnormal discharge can be preventedeffectively.

A plasma device according to the present invention is characterized bycomprising a susceptor for placing a target object thereon, a processingvessel for accommodating the susceptor, exhaust means for evacuating aninterior of the processing vessel, gas supply means for supplying a gasinto the processing vessel, and antenna means which is arranged tooppose that surface of the susceptor where the target object is to beplaced and which supplies a microwave into the processing Vessel,wherein the radial antenna described above is used as the antenna means.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing the arrangement of an etching apparatusaccording to the first embodiment of the present invention;

FIG. 2 includes views showing the arrangement of an example of theradial antenna and the distribution of its electric field radiation;

FIG. 3 includes views showing the shapes of slots formed in the radialantenna shown in FIG. 2;

FIG. 4 includes plan views showing the arrangements of examples of aconductive plate that forms a radiation surface;

FIG. 5 is a sectional view showing a modification of ridges;

FIG. 6 includes views showing the arrangement of another example of theradial antenna and the distribution of its electric field radiation;

FIG. 7 includes views showing the arrangement of an example of a radialantenna conventionally used in a microwave plasma device, and thedistribution of its electric field radiation; and

FIG. 8 is a view showing the shape of a slot formed in the radialantenna shown in FIG. 7.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention will be described in detailwith reference to the drawings. A description will be made by way ofexamples in which a plasma device using a radial antenna according tothe present invention is applied to etching apparatuses.

First Embodiment

FIG. 1 is a view showing the arrangement of an etching apparatusaccording to the first embodiment of the present invention. In FIG. 1,the sectional structure of part of the arrangement is shown.

The etching apparatus shown in FIG. 1 has a cylindrical processingvessel 11 with an upper opening. This processing vessel 11 is made of aconductive member such as aluminum.

Exhaust ports (exhaust means) 14 communicating with a vacuum pump (notshown) are formed in the bottom of the processing vessel 11, so theinterior of the processing vessel 11 can be evacuated to a predeterminedvacuum degree.

A plasma gas supply nozzle 15 for introducing a plasma gas such as Arinto the processing vessel 11, and a process gas supply nozzle 16 forintroducing an etching gas are formed in the upper and lower portions,respectively, of the side wall of the processing vessel 11. The nozzles(gas supply means) 15 and 16 are formed of quartz pipes or the like.

The processing vessel 11 accommodates a susceptor 22 for placing anetching target substrate (target object) 21 on its upper surface. Thesusceptor 22 is fixed on a support table 23 which is fixed to the bottomof the processing vessel 11 through an insulating plate 24. Thesusceptor 22 is also connected to a bias RF power supply 26 through amatching box 25.

A flat plate-like dielectric plate 13 is horizontally arranged in theupper opening of the processing vessel 11. Silica glass or a ceramicmaterial (e.g., Al₂O₃ or AlN) with a thickness of about 20 mm to 30 mmis used to form the dielectric plate 13. A seal member 12 such as anO-ring is disposed at the bonding portion between the processing vessel11 and dielectric plate 13. This assures the hermeticity in theprocessing vessel 11.

A radial antenna 30 is disposed on the dielectric plate 13 with itsradiation surface (conductive plate 31 to be described later) facingdown. The radial antenna 30 is an antenna means that supplies amicrowave MW into the processing vessel 11 through the dielectric plate13. The dielectric plate 13 is opposed to the radiation surface of theradial antenna 30, and covers the radiation surface entirely. Hence, theradial antenna 30 is protected from a plasma generated in the processingvessel 11. The circumferential surfaces of the dielectric plate 13 andradial antenna 30 are covered by a shield material 17.

FIG. 2 includes views showing the arrangement of the radial antenna 30and the distribution of its electric field radiation, in which FIG. 2(a) is a plan view showing the radiation surface of the radial antenna30, FIG. 2( b) is a sectional view taken along the line IIb-IIb′ of FIG.2( a), and FIG. 2( c) is a conceptual view showing the distribution ofthe electric field radiated by the radial antenna 30. FIG. 2( a) isshown conceptually to clarify the characteristic feature of the presentinvention. In FIG. 2( c), the axis of abscissa represents the distancefrom the center of the radial antenna 30 in the radial direction, andthe axis of ordinate represents the strength of the electric fieldradiated from the radial antenna 30.

As shown in FIG. 2( b), the radial antenna 30 is formed of the firstconductive plate 31 constituting the radiation surface, a secondconductive plate 32 arranged above the conductive plate 31 to oppose it,and a ring member 34 for connecting the peripheral edges of theconductive plates 31 and 32. The ring member 34 holds the gap betweenthe conductive plates 31 and 32 to dl. The radial antenna 30 with thisarrangement has a hollow cylindrical shape. The two conductive plates 31and 32 form a radial waveguide 33 which guides the microwave MW. Theconductive plates 31 and 32, and ring member 34 are made of a conductorsuch as copper or aluminum.

A microwave inlet 35 through which the microwave MW is introduced isformed at the center of the conductive plate 32. A large number of slots36 extending in the circumferential direction are concentrically formedin the conductive plate 31 which forms the radiation surface, as shownin FIG. 2( a).

FIG. 3 includes views showing examples of the shapes of the slots 36.The shapes of the slots 36 may be rectangular as shown in FIG. 3( a), orarcuate as shown in FIG. 3( c). The four corners of each of the slots 36shown in FIGS. 3( a) and 3(c) may be rounded as shown in FIG. 3( b) and3(d). In each of the slots 36 shown in FIGS. 3( a) to 3(d), the size inthe longitudinal direction, i.e., the slot length, is defined as L1 andthe size in the widthwise direction, i.e., the width, is defined as W1.

As shown in FIG. 2( a), the slot lengths L1 of the slots 36 increase inprinciple as the slots 36 are further away from the center from theconductive plate 31. Note that in the radial direction, the slot lengthsL1 are discontinuous across the boundary between a region A1 that doesnot oppose ridges 37 (to be described later) and a region A2 thatopposes them. Assuming that the wavelength of the microwave MW in theradial waveguide 33 is λg, the slot length L1 is set sufficientlysmaller than a length corresponding to λg/2 at maximum. In this radialantenna 30, the maximum slot length L1 is set to λg/3.

The width W1 of each slot 36 is set to about 2 mm considering theinfluence on the electromagnetic field in the radial waveguide 33 andthe like.

The pitch among the slots 36 in the radial direction is set on the basisof λg. To realize a radiation type antenna as shown in FIG. 4( a), thepitch is set to about a length corresponding to λg. To realize a leakantenna as shown in FIG. 4( b), the pitch is set to about a lengthcorresponding to λg/20 to λg/30.

As shown in FIG. 1, a coaxial cable 41 is connected to the center of theradial antenna 30. An outer conductor 41A of the coaxial cable 41 isconnected to the microwave inlet 35 of the conductive plate 32. Thedistal end of a central conductor 41B of the coaxial cable 41 forms acircular cone, and the bottom of the circular cone is connected to thecenter of the conductive plate 31.

The coaxial cable 41 connected to the radial antenna 30 in this manneris connected to a microwave generator 45 through a rectangular coaxialconverter 42 and rectangular waveguide 43. The microwave generator 45generates the microwave MW of, e.g., 2.45 GHz. The frequency of themicrowave MW suffices if it falls within the range of 1 GHz to 10-oddGHz. A matching circuit 44 for impedance matching is connected midwayalong the rectangular waveguide 43, so that the power use efficiency canbe improved.

As shown in FIG. 2( b), in the radial waveguide 33, a plurality ofridges 36 are fixed to the lower surface of the conductive plate 32.These ridges 37 are made of a conductor such as copper or aluminum, andare fixed with machine screws from the conductive plate 36 side. Theridges 37 are quadrangular prismatic members with a height h smallerthan d1, and operate as adjusting members for adjusting the distance upto the upper surface of the conductive plate 31. Of the edges of eachridge 37, an edge 37E which is the closest to the center of the radialwaveguide 33 is chamfered as shown in FIG. 2( b).

As shown in FIG. 2( a), the ridges 37 are disposed radially in thatregion A2 which is away from the center of the conductive plate 32 by apredetermined distance or more and where the radiated electric fieldstrength is to be increased. At this time, the ridges 37 are disposed ona region opposing the slots 36. As shown in FIGS. 4( a) and 4(b), whenslots 36A and 36B are formed in the entire region of the conductiveplate 31 excluding the central portion, the ridges 37 are arrangedcontinuously in the circumferential direction.

At the region A1 where no ridges 37 are disposed, the height of theradial waveguide 33 corresponds to the distance dl from the conductiveplate 32 to the conductive plate 31. In contrast to this, at the regionA2 where the ridges 37 are disposed, the height of the radial waveguide33 corresponds to a distance d2 (=d1−h (<d1)) from the ridges 37 to theconductive plate 31. At the region A2 where the height of the radialwaveguide 33 is small due to the presence of the ridges 37, the powerdensity increases. Therefore, at the region A2 provided with the ridges37, the radiated electric field strength can be increased even when theslot length L1 is decreased to be sufficiently smaller than a lengthcorresponding to λg/2.

Resonance can be suppressed if the slot length L1 is decreased to besufficiently smaller than a length corresponding to λg/2. Thus, even ifthe width W1 of each slot 36 is as small as 2 mm, induction of abnormaldischarge can be prevented. More preferably, when L1<λg/3, induction ofabnormal discharge can be prevented effectively.

Furthermore, when the slot length L1 of each slot 36 is adjusted inaccordance with the distance from the center of the conductive plate 32so that the radiated electric field strength is controlled, an electricfield radiation distribution as shown in, e.g., FIG. 2( c), can beobtained without inducing abnormal discharge.

A delay member made of a dielectric material such as a ceramic materialwith a relative dielectric constant larger than 1 may be arranged in theradial waveguide 33. When the delay member is used, the electric fieldradiation efficiency can be improved.

The operation of the etching apparatus shown in FIG. 1 will bedescribed.

The substrate 21 is placed on the upper surface of the susceptor 22, andthe interior of the processing vessel 11 is set to a vacuum degree of,e.g., about 0.01 Pa to 10 Pa. While maintaining this vacuum degree, Aris supplied as the plasma gas from the plasma gas supply nozzle 15, andan etching gas such as CF₄ is supplied from the process gas supplynozzle 16 under flow rate control.

With the plasma gas and etching gas being supplied into the processingvessel 11, the microwave MW from the microwave generator 45 is suppliedto the radial antenna 30 through the rectangular waveguide 43,rectangular coaxial converter 42, and coaxial cable 41.

The microwave MW supplied to the radial antenna 30 propagates radiallyfrom the center toward the periphery of the radial waveguide 33 formedof the conductive plates 31 and 32. At this time, the microwave MW isradiated gradually from the slots 36, so the power of the microwave MWpropagating in the radial waveguide 33 decreases gradually toward theperiphery of the radial waveguide 33. At the region A2 which is awayfrom the center of the conductive plate 32 by the predetermined distanceor more and on which the ridges 37 are disposed, however, the height ofthe radial waveguide 33 changes from d1 to d2 with a decreasecorresponding to the height h of the ridges 37, so the power density inthe radial waveguide 33 does not become so small as in the conventionalcase. Hence, even when the slot length L1 is shorter than a lengthcorresponding to λg/3, the electric field strength of the microwave MWradiated from all the slots 36 becomes sufficiently large.

On the other hand, since L1<λg/3, even if the width W1 of each slot 36is as small as 2 mm, abnormal discharge is not induced. Therefore, theportion around the slot 36 is not distorted or dissolved by dischargeunlike in the conventional case.

The microwave MW radiated from the radial antenna 30 is transmittedthrough the dielectric plate 13 and is introduced into the processingvessel 11. The microwave MW forms an electric field in the processingvessel 11 to ionize Ar, thus producing a plasma in a space S1 above thesubstrate 11 as the processing target.

In this etching apparatus, since the susceptor 22 is biased with anegative potential, ions are extracted from the produced plasma to etcha substrate 21.

The radial antenna 30 shown in FIG. 2 uses the ridges 37 with theconstant height h. Alternatively, as in a radial antenna 30A shown inFIG. 5, ridges 37A with a height that changes continuously in the radialdirection may be disposed on the lower surface of the conductive plate32, so that the lower surface opposing the conductive plate 31 inclines.At this time, the inclination of the ridges 37A is designed byconsidering the impedance matching inside and outside the radial antenna30A.

Second Embodiment

In the radial antenna 30 shown in FIG. 2, the ridges 37 are disposedradially. Alternatively, the ridges may be disposed along the peripheryof the conductive plate 32. FIG. 6 includes views showing thearrangement of a radial antenna in which ridges are disposed in thismanner. FIG. 6( a) is a plan view showing the radiation surface of theradial antenna, and FIG. 6( b) is a sectional view taken along the lineVIb-VIb′ of FIG. 6( a). FIG. 6( a) is shown conceptually to clarify thecharacteristic feature of the present invention. In FIG. 6, the sameportions as in FIG. 2 are denoted by the same reference numerals, and adescription thereof will be omitted as required.

As shown in FIG. 6, three ridges 137A, 137B, and 137C serving asadjusting members are fixed along the periphery of the lower surface ofa conductive plate 32 in a radial waveguide 133. The ridges 137A to 137Cform concentric circles conforming to the periphery of the conductiveplate 32, and are disposed in this order from the inner side. Assumingthat the heights of the ridges 137A, 137B, and 137C are h1, h2, and h3,respectively, h1<h2<h3 holds. Namely, the ridges 137A to 137C becomehigher as they are further away from the center of the conductive plate32. Hence, the radial waveguide 133 becomes lower as it is further awayfrom the center of the radial waveguide 133. This can change the powerdensity in the radial waveguide 133 in the radial direction.

Therefore, in the same manner as in the radial antenna 30 shown in FIG.2, even when a slot length L1 is set to a length sufficiently smallerthan a length corresponding to λg/2, e.g., to be sufficiently smallerthan a length corresponding to λg/3, if the ridges 137A to 137C areprovided, the radiated electric field strength can be increased. Evenwhen a width W1 of each slot 36 is as small as 2 mm, induction ofabnormal discharge can be prevented.

Having described a case in which the three ridges 137A to 137C aredisposed concentrically, the number of ridges is not limited to three.

While the heights h1, h2, h3 of the respective ridges 137A, 137B, and137C are each constant, they may change continuously in the radialdirection in the same manner as in the ridges 37 shown in FIG. 5.

As has been described above, in the above embodiments, adjusting membersfor adjusting the distance from the second conductive plate to the firstconductive plate, which two conductive plates forming the radialwaveguide, are provided. At the periphery of the radial waveguide wherethe power density conventionally decreases, the power density can beincreased by decreasing the height of the radial waveguide. Even whenthe slot length is decreased to be sufficiently smaller than a lengthcorresponding to λg/2, the radiated electric field strength can beincreased. Hence, when adjusting members are provided to correspond to aregion where the radiated electric field strength is to be increased, adesired electric field distribution can be obtained without inducingabnormal discharge.

The plasma device using the radial antenna according to the presentinvention can be applied not only to the etching apparatus describedabove but also to other plasma devices such as a plasma CVD apparatus orashing apparatus. The application of the radial antenna according to thepresent invention is not limited to the plasma device described above.For example, the radial antenna according to the present invention maybe applied to a communication antenna, particularly, adistributor/synthesizer which distributes and synthesizes atransmission/reception signal of an array antenna.

1. A radial antenna comprising: a first conductive plate in which aplurality of slots are formed, a second conductive plate in which amicrowave inlet is formed at a central portion thereof, the secondconductive plate disposed opposite to said first conductive plate, aring member for connecting peripheral edges of said first and secondconductive plates, and a conductive adjusting member attached to thesecond conductive plate in a region excluding the central portion ofsaid second conductive plate in which the microwave inlet is formed, theconductive adjusting member disposed in a radial waveguide formed bysaid first and second conductive plates such that a distance between theconductive adjusting member and said first conductive plate is smallerthan a distance between the second conductive plate and the firstconductive plate; wherein the conductive adjusting member comprises anelectrically conductive adjusting member.
 2. A radial antenna accordingto claim 1, characterized in that said conductive adjusting member isdisposed at an outer peripheral portion of said second conductive plate.3. A radial antenna according to claim 1, characterized in that saidadjusting member is disposed along the peripheral edge of said secondconductive plate.
 4. A radial antenna according to claim 1,characterized in that said adjusting member is set to become higher asbeing further away from a center of said second conductive plate.
 5. Aradial antenna according to claim 1, characterized in that the slots aresubstantially rectangular or arcuate, and a slot length of the slots issmaller than a length corresponding to ⅓ the wavelength of a microwavepropagating in said radial waveguide.
 6. A plasma device characterizedby comprising a susceptor for placing a target object thereon, aprocessing vessel for accommodating said susceptor, exhaust means forevacuating an interior of said processing vessel, gas supply means forsupplying a gas into said processing vessel, and antenna means which isarranged to oppose that surface of said susceptor where the targetobject is to be placed and which supplies a microwave into saidprocessing vessel, wherein said antenna means comprises a radial antennaaccording to claim
 1. 7. A radial antenna according to claim 1, whereinthe conductive adjusting member is configured to increase a powerdensity at a peripheral portion of the radial waveguide.
 8. A radialantenna according to claim 1, characterized in that a plurality of saidconductive adjusting members are disposed radially when viewed from top.